Flash tower and flash distillation



March 1956 T. P. BINDER ETAL ,73

FLASH TOWER AND FLASH DISTILLATION Filed April 5. 1953 DEM/STER /2 /20 Feed In/ef A A Feed ln/ef S TR/PPER GH/M/VEY" 27 Sleam INVENTORS Thomas F. Binder Thomas 6. 0arro// Thomas P. Binder and Thomas Carroll, T exas City,

Ten, assignors, by mes'ne assignments, to The American Oil Company, Texas City, Tex., a corporation of Texas Application April 3, 1953,'Serial No. 346,746

1 Claim. (Cl. 261-114) This invention relates to flash towers and flash distillation. More particularly the invention relates to the flash distillation of a hydrocarbon mixture containing a relatively small amount of bottoms product.

In the flash distillation art the design of the flash tower represents a compromise between efficient fractionation of the vaporized fraction and eflicient stripping of the liquid bottoms fraction. The problem is particularly aggravated when the feed to the flash tower contains only a relatively small amount of liquid bottoms fraction, e. g., in the flash distillation of a crude or reduced crude which contains only a small amount such as about volume percent of residuum; or in the flash distillation v of the light tar derived from gas oils.

When only a small amount of residuum (bottoms) is to be produced and said residuum must be stripped of the low boiling materials in order to obtain a satisfactory flash point, the tower must provide :eflicient stripping of this residuum. The large quantity of vaporizable matethe thermal cracking of rial requiresa towerof large diameter and considerable height. The large diameter of the tower seriously interferes with eflicient operation in the residuum section of the tower, i. e., that portion of the tower below the feed inlet. The provision of bubble trays therein results in excessive maintenance owing to the coking or" the residuum on the trays because of the prolonged exposure of the pool of liquid on the trays to the high temperature in the tower. Also, the stripping isrelatively inefficient because of the small amount of liquid passing over the tray in relationship to the amount of stripping medium introduced at the bottom of the tower. Omission of bubble trays requires a relatively deep pool of liquid and increased residence time for a satisfactorily stripped product. This results in considerable coking and side reactions in the tower bottoms.

A method commonly used in this sort of situation involves the reduction in diameter of the tower below the point of feed inlet. Still another method involvesv the use of an external stripper wherein the residuum is Withdrawn from the tower, stripped in the external stripper and the stripper overheadis reintroduced intothe tower. These methods provide eflicient stripping at the expense of excessive tower heights to gain the residuum volume necessary for proper pump holding time. Increased space and supporting structures are required.

An object of this invention is a flash tower for the flash distillation of a feed containing a relatively small amount of bottoms product. Another object is a flash tower which minimizes carryover of bottoms product into the vaporized product. A particular object is a tower for flash distillation of a mixture of hydrocarbons which contains a relatively small amount of liquid bottoms product. Another particular object is a tower for flash distillation of crude oil or reduced crude which contains only a relatively small amount of residuum. Other objects will become apparent in the course of the detailed description of the invention.

- opening and attached to said plate.

ice

flash tower of this invention. Many details of constructio'n have been omitted as these may readily be added by one skilled in the art. The lower portion the drawing is a vertical cross-sectional view and the upper portion a wholly schematic view of the external portion of the tower.

In the drawing the numeral 11 indicates the shell of the flash tower. Shell 11 is providedwith ingress and egress openings such as inlets for the feed, stripping medium, manways for maintenance workers and outlets for the withdrawal or various product fractions. In this embodiment several feed inlets are utilized to distribute the feed around the circumference of the tower. Two of these feed inlets l2 and 120 are shown. The tower contains an inlet chimney which consists of a doughnut plate 13 and a chimney 14. The doughnut plate 13 is attached to the shell 11 in spaced vertical relationship above the feed inlet openings. The juncture of the doughnut plate 13 and shell 11 is vapor-tight. Chimney 14 is positioned below doughnut plate 13 at the inner portion of the annulus and extends to a point in spaced relationship below the feed inlet opening. In an actual commercial-size flash tower: The tower height is ft.; the internal diameter is 12ft. The annulus of doughnut platelS is 2 ft; and the height of chimney 14 is 2 ft. 6 in. The feed inlet point is approximately at the vertical mid-point of chimney 14.

v A demister means 16 is positioned in spaced vertical relationship above the feed ingress opening and also above "doughnut plate 13.

"metal screening, In this illustration a York mesh dernister, about 6 in. in thickness, is used.

in the upper portion of the flash tower liquid-vapor contacting means areprovided' to fractionate the vapors into various product fractions. In this illustration conventional bubble trays are used. Only two bubble cap trays, namely 18 and 19, shown. The drawing shows schematically trapouts for the withdrawal of several fractions. These withdrawal means are illustrated as lines 21, 22, 23 and 24. Obviously provision for the Withdrawal of more fractions or fewer fractions is within ordinary skill of the art.

An internal stripper 2-7 is positioned in spaced relationship below the feed inlet openings 12 and 12a. This internal stripper 27 consists of (a) a plate attached to the circumference of said shell in a vapor-tight fit; this plate 31 is provided with an opening 32. This opening 32 leads to (b) a chimney 33 positioned below said Chimney 33 is provided with (c) vapor-liquid contacting means 34, 34a and 35th. v

In the illustration plate 31 is doughnut-shaped with an annulus of 4 ft. The hole of the doughnut is reduced in size by means of a baffle 36. Baffle 36 thus is shaped like a circle with a segment missing. Opening 32 is a segment of a circle having a rise of 1 ft. Herein baflle 36 is imperforate. However, it may be desirable in some methods of operation to employ a perforated baffle.

Liquid-vapor contacting means 34, 34a and 3415 may be of any conventional form such as bubble trays, di c and doughnut trays or various packings. In this illustration the liquid-vapor contacting means consist of modified cascade plates that cause the ascending vapor to pass through sheets of descending liquid.

The internal stripper chimney in this illustration is 4 ft. in diameter and has a height of 6 ft. 6 in. Thus sheared :Mar. 2a, lass means 38 byway of ingress line 39.

The operation of the fiash tower of this invention is described as follows: The feed may be total crude oil or reduced crude or light tar from the cracking of gas oil or a very wide boiling fraction of gas oil. in general any feed which contains a relatively small amount of high boiling material which is preferably removed as a bottoms product, and which is susceptible to thermal decomposition at distillation temperatures, may be utilized. in this illustration the feed is a naphtheniotype Texas crudc. The crude is heated in a furnace not shown to a temperature high enough to vaporize about 90% of it by self-contained heat when the crude is passed into the to\ or by way of inlet lines 12 and 120.

he flash tower is operated at about atmospheric pressure and the bottoms temperature will be about 700 F. The term liquid bottoms product or residuum or residual material is intended to mean a material boiling above about 750 F. at normal pressures, i. e., about atmospheric pressure. in order to avoid thermal decomposition of these materials, it is preferred to operate at about a maximum of 700 F. in order to keep thermal decomposition of the residuum to a minimum. As the feed enters the tower it flashes to form a mixture of vapor and liquid; this mixture is deflected by chimney 14 and is forced to make a three-quarter turn in order to pass up the tower height. These changes of direction cause a cons derable amount of the entrained liquid particles to be thrown downwards in the direction of plate 31.

The vapors and entrained liquids pass upwards through demistcr 16. Demister 16 coalesces the liquid particles to droplets too large to be carried upwards by the vapors.

These coalesced droplets drop out of demister 16 and...

pass through the inlet chimney to plate 31. The vapors above demister 16 are substantially free of entrained high boiling liquids. These vapors are fractionated by means of' the bubble trays in the upper portion of the tower. A heavy gas oil, a light gas oil and a heavy naphtha fraction are withdrawn separately by way of lines 21, 22 and 23 and are passed to further processing not shown. A combined light naphtha and gas fraction is withdrawn by way of line 24 and is sent to a stabilizing operation not shown.

The presence of demister 16 in this flash tower essentially overcomes one of the most serious problems of lash distillation, i. e., entrainment of residuum resulting in contamination of the distillate fractions. Also, the coking of residuum particles retained on the bubble trays with consequent high maintenance costs is essentially eliminated.

The liquid material from demister 16 and directly from the feed is directed by plate 31 and baffle 36 into opening 32. This liquid cascades through opening 32 and over the contact trays 34b, 34a and 34. Owing to the reduction in cross-sectional area of internal stripper 27 to stripper, positioned in spaced relationship below said feed about one-ninth the total cross-sectional area of shell 11, a high liquid rate is maintained in internal stripper 27 in spite of the fact that the residuum amounts to only about 7-10 volume percent of the feed.

Steam from line 39 passes through distributor 38 into the tower below plate 31 and preferably below the lower end of chimney 33. The steam passes up through chimney 33, and is intimately contacted with the descending sheets of residuum. Stripping steam and the lower boiling materials present in the residuum at this point pass out 'of chimney 33 by way of opening 32. Any entrained 56651 1110111 is removed from this vapor stream by demister Stripping steam distributor 38 is positioned above the pool of residuum maintained in the bottom of shell 11. The depth of this pool is determined by pump holding time requirement although it is preferred to operate with a shallow pool, i. e., about 1 ft. in depth. The stripping steam is able to remove additional quantities of lower boiling materials from the pool of residuum, but owing to its entry above the surface of the pool, no violent agitation occurs; thereby the stable foam of steam and residuum which is frequently formed in towers is avoided. What little foam is formed is efiectively eliminated in internal stripper 27. Stripped high flash point residuum product is withdrawn from the bottom of the fiash tower by way of line 41 and is passed to storage not shown. Thus having described the invention, what is claimed is:

A flash tower comprising a shell provided with ingress vapor-liquid contacting means positioned above said demrster means, a'doughnut plate positioned in spaced 'vertical' relationship between said demister means and said feed ingress opening, which plate is provided with .an inlet chimney extending between said doughnut plate and a point in space below and in spaced horizontal relationship from said feed ingress opening and affords a vapor tight fit between said plate and said shell, an internal ingress opening and said inlet chimney, said stripper consisting of (a) a plate attached to said shell to afford a vapor tight fit and provided with an opening, (b) a stripper chimney positioned below said plate opening and attached to said plate, and (c) vapor-liquid contacting means positioned in said stripper chimney, and means for introducing stripping medium positioned in vertical spaced relationship between the shell and the lower end of said stripper chimney.

References Cited in the file of this patent UNITED STATES PATENTS 1,655,998 Gilmore Jan. 10, 1928 2,054,096 Potts et al Sept. 15, 1936 2,084,768 Garrison June 22, 1937 2,095,418 McConnell Oct. 12, 1937 2,105,935 Swanson Jan. 18, 1938 2,151,310 Aldridge et al. Mar. 21, 1939 2,217,356 Becknell Oct. 8, 1940 2,658,863 Guala Nov. 10, 1953 

